CGSS 2023 Chemistry Prelim P2 Answers

1 Section A A1 (a) calcium chloride (b) ammonium chloride (c) hydrogen chloride (d) aluminium chloride (e) hydrogen chloride A2 (a) (i) The catalyst provides an alternative pathway of lower activation energy for the reaction to occur. More molecules/particles have energy greater than or equal to the activation energy. This leads to a higher frequency of effective collisions, leading to a greater rate of reaction. [1] role of a catalyst [1] explanation in terms of collisions and energy Discussion: • Reject: catalyst ‘lowers Ea’ more molecules have the minimum Ea (doesn’t cover the idea of greater than or equal to; Ea is an indication of energy, not energy itself) • Ignore: Greater frequency of collisions (ii) Catalysts are not chemically changed/regenerated at the end of the reaction so they can be reused infinitely. Hence, no additional cost is needed to purchase more new catalysts after each process. Catalysts provide an alternative pathway with lower activation energy for the reaction to occur, hence the reaction consumes less energy and lowers costs. [1] each - show an understanding of the workings of a catalyst: • regenerated at the end of the reaction, • requires less energy to operate since the reaction occurs with a lower Ea Note: student just needs to mention alternative pathway of lower Ea in (i) Discussion: • Reject: catalyst allows more hydrogen to be produced within a shorter period of time (does not relate to cost) unless they explain further about reduced manpower costs/labour costs only a small amount is needed (not about the quantity, but the fact that this small amount is regenerated and hence reused) (counter example: purchasing a catalyst requires a higher expenditure compared to when no catalyst is used) to include in Markers’ report: • Students need to write in a concise manner to avoid contradicting themselves in lengthy answers

(b) The reaction removes carbon monoxide/CO. [1] It is important to remove carbon monoxide , which is toxic/ poisonous/dangerous. I t reduces the ability of haemoglobin/red blood cell/blood to transport/carry oxygen to the rest of the body [1] [1] highlight what Stage 2 removes [1] impact of carbon monoxide on human health (don’t need to mention toxicity) Discussion: • mark allocation 1st mark: purpose of reaction 2nd mark: impact of CO on human health • allow: any other that CO causes reduced oxygen carrying ability (accounts for explain) (TB pg 394) eg. “CO reduces the amount of oxygen in the body” eg. “Lowers the amount of oxygen in the blood” • reject: no reference to oxygen carrying ability “CO can cause breathing problems and death in humans” (does not account for ‘explain’, but states the ‘effect’) Follow up: • check if ‘toxicity’ is required in the answer (2018/2/6b) Markers’ report implies that students need to elaborate on its toxicity Additional point for 2024 onwards (old syllabus uses ‘poisonous’) (c) Updated: ‘change from +1 in H2O to 0 in H2’ ‘change from +1 to 0’ ‘decreases from +1 to 0’ Discussion: • ‘change’: if it is colour change, initial colour and final colour

• ‘change in oxidation state’: if it is numerical? (ref: 2012/2/3bi, 2022/2/6bii) From 2022: Marker’s report implies that students need to work out both initial and final oxidation states From 2012: Marker’s report implies that students need to work out both initial and final oxidation states (d) 2H2 + O2 → 2H2O [1] Only water is produced from the combustion of hydrogen, which does not cause pollution. [1] [1] chemical equation for the combustion of hydrogen [1] comment on water formed which is harmless [need to relate the product] Reject: • No “harmful products” are produced (e) No. of moles of hydrogen = 1000 2 = 500 mol [1] Enthalpy change in kJ/kg = −286 x 500 = −143 000 kJ/kg [1] [1] calculate no. of moles of hydrogen from the given mass [1] calculate enthalpy change in kJ/kg Note: no penalty if no units are given, but annotate on the script Include in markers’ report: • Need to have clear statements and working

A3 (a) I−(aq) + Ag+(aq) → AgI(s) [1] correct equation [1] state symbols Accept: • State symbols all correct if correct chemical equation is given (b) Step 2: Iron is more reactive than silver. Iron displaced silver from silver iodide to form iron(II) iodide and silver (product A) Step 3: Chlorine is more reactive than iodine. Chlorine displaced iodine from iron( II) iodide to form iodine and iron(II) chloride (product B). [1] relative reactivity of iron and silver [1] explain what has been displaced and identify product A [1] relative reactivity of chlorine and iodine [1] explain what has been displaced and identify product B Note: • Reject: comparison between reactivity of halogen and halides (eg. chlorine is more reactive than iodide) • Reject: incorrect terms (eg. …displaced iodide from…) • Reject: “greater distance in reactivity”, “higher up in the reactivity series” (c) Sublimation • Reject: “heating”

A4 (a) metal positive or negative electrode cathode copper positive anode zinc negative [1] one mark for any two boxes (b) The size of the copper discs will increase and the size of zinc discs will decrease. [1] Zinc will be oxidised to form zinc ions as it is more reactive than copper. [1] Copper(II) ions will be reduced/preferentially discharged/gain electrons to form copper. [1] [1] describe the effect on mass of individual discs [1] state what is oxidised and explain based on relative reactivity of metals (not about relative position in the reactivity series) [1] state is reduced and explain based on the ease of discharge based on the electrochemical series for cations Reject: • “Zinc is discharged” Discussion: • If students mix up metals at the electrodes, they would have the wrong analysis of the chemistry behind it • But – award mark for specific chemistry (Eg. copper(II) ions being preferentially discharged) (c) There is a greater difference in reactivity between silver and zinc compared to between copper and zinc. [1] compare between Zn and Ag vs Zn and Cu; not between Cu and Zn Discussion: • Allow: Silver is less reactive than copper. Greater difference in reactivity between silver and zinc ‘Further apart in reactivity’, ‘greater distance between the metal reactivity series’ • Reject: ‘higher or lower in the reactivity series’ instead of ‘relative reactivity’ “silver is lower than zinc in the reactivity series” “silver is lower in the reactivity series” (d) (i) Positive electrode: 2Cl−(aq) → Cl2(g) + 2e− [1] Negative electrode: 2H+(aq) + 2e− → H2(g) [1] Discussion: • Missing or incorrect state symbols or “aqueous electrons” (-1 overall) (ii) The volume of gases collected will be the same. In the overall equation, the molar ratio of chlorine gas to hydrogen gas formed is the same/ratio of chlorine gas: hydrogen gas = 1:1 [1] conclusion and explanation on the ratio of products formed

Allow: because implies that the same ratio of products would be formed • mole ratio of electrons in half-equation are the same Reject: incorrect because they should focus on the products • mole ratio of chloride ions and hydrogen ions is the same • ratio of H to Cl in concentrated hydrochloric acid is 1:1

(iii) Chlorine gas is soluble in water OR some chlorine gas dissolved in water [1] answers must relate to solubility of chlorine in water. Allow: • soluble/dissolved in acid/electrolyte (because links to context) Reject: • chloride gas • soluble in solvents (too generic; not linked to question) • chlorine gas is more dense/diffuse out (iv) The white precipitate is silver chloride/Ag